L ENG COWLING FELL OFF AFTER TKOF.

Narrative

WHILE PREFLTING; I FOUND A PROB WITH THE NOSE STOUT. I CALLED MAINT AND CONTINUED MY PREFLT. A THOROUGH PREFLT WAS ACCOMPLISHED UNTIL THE L NACELLE (STARTED WITH NOSE OF ACFT AND WENT CLOCKWISE). WHILE CHKING EXHAUST STACK (L SIDE); AND PROP I WAS SIDETRACKED INTO GOING BACK TO THE NOSE WHERE THE MECH WAS WORKING ON THE PROB PREVIOUSLY MENTIONED. AFTER CHKING TO SEE WHAT I SHOULD BE LOOKING FOR; ON THE NOSE STOUT; THROUGH THE DAY (IE; POSSIBLE PROBS); I WENT BACK TO THE L-HAND NACELLE TO COMPLETE THE PREFLT. I DON'T RECALL GOING BACK TO THE L (OUTBOARD NACELLE) SIDE LIKE I SHOULD HAVE. INSTEAD; I WENT TO THE INBOARD L NACELLE - ALL APPEARED NORMAL. THE LOCKS FOR THE UPPER COWLING ARE A 90 DEG TURN HEXAGONAL STEM ABOUT 3/4 INCH LONG (WHEN VIEWED FROM THE SIDE OF THE COWLING). THERE WAS VERY LITTLE IN THE WAY OF MARKINGS ON THE LOCKS TO CONFIRM THEIR LOCKING POS. NORMALLY; THERE IS A HORIZ LINE ON THE FACE - OR AN ARROW TO MATCH UP WITH A REF LINE ON THE COWL. HOWEVER; ON THIS ACFT; IT WAS VERY DIFFICULT TO SEE ANY OF THE MARKINGS. A MUCH BETTER TEST WOULD TO HAVE BEEN TO STRIKE THE COWL WITH AN UPWARD BLOW OF MY PALM TO JAR THE COWLING AND SEE IF IS WAS SECURE; I DID NOT. WE TOOK OF (NO PAX); AND LATER NOTICED THE COWL MISSING. RETURNED FOR LNDG; AND FOUND NONE OF THE (4) BLOCKS HAD BEEN SECURED. THE ACFT FLEW NORMALLY AND WE DID NOT NOTICE ANY DIFFERENCES IN PERFORMANCE; LNDGS; ETC. THE DIRECTOR OF OPS; FAA FSDO (ALBANY); AND DIRECT OF MAINT WERE NOTIFIED. THE OUTCOME WAS AS FOLLOWS: ME; INCIDENT RPT WITH ALBANY FSDO REGARDING SIT. LETTER ON FILE OF EMPLOYER; STATING I RECEIVED REMEDIAL TRAINING ON FASTENING COWL LOCKS. MECH - POSSIBLE VIOLATION FOR FAILURE TO LOCK DOWN COWLING. AFTER MAINT WAS PERFORMED. LETTER ON FILE BY EMPLOYER STATING SIT. REGARDING HUMAN FACTORS; ETC; I BELIEVE THERE WERE 2 MAJOR FACTORS INVOLVED. 1) THERE WAS NO VISUAL INDICATOR THAT NOTIFIED A POTENTIAL PROB. IE; ZEUS (NOT SURE OF SPELLING) FASTENERS STICK OUT WHEN THEY ARE NOT SECURED. THE SAME APPLIES TO NORMAL LATCHES - IT IS OBVIOUS WHEN THEY ARE NOT SECURED. IN THIS CASE; IT IS EASY TO OVERLOOK A FASTENER WITH LITTLE OR NO MARKING ON IT. THE ONLY TRUE TEST IS TO STRIKE THE COWL TO CHK FOR SECURITY. THIS APPEARS TO BE A POOR DESIGN. 2) I LET MYSELF GET SIDETRACKED MIDWAY THROUGH PREFLT. I SHOULD HAVE RETURNED TO A POINT WELL BEFORE WHERE I KNEW I LEFT OFF.